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Effect of sire breed (Southdown,Suffolk), sex, and growth path on carcasscomposition of crossbred lambsA. H. Kirton a , G. L. Bennett a b , J. L. Dobbie a , G. J. K. Mercer a &D. M. Duganzich aa AgResearch, Ruakura Agricultural Centre , Private Bag 3123,Hamilton, New Zealandb Clay Centre , P. O. Box 166, Nebraska, 68933, United StatesPublished online: 17 Mar 2010.

To cite this article: A. H. Kirton , G. L. Bennett , J. L. Dobbie , G. J. K. Mercer & D. M.Duganzich (1995) Effect of sire breed (Southdown, Suffolk), sex, and growth path on carcasscomposition of crossbred lambs, New Zealand Journal of Agricultural Research, 38:1, 105-114, DOI:10.1080/00288233.1995.9513109

To link to this article: http://dx.doi.org/10.1080/00288233.1995.9513109

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New Zealand Journal of Agricultural Research, 1995, Vol. 38: 105-1140028-8233/95/3801-0105 $2.50/0 © The Royal Society of New Zealand 1995

105

Effect of sire breed (Southdown, Suffolk), sex, and growth path oncarcass composition of crossbred lambs

A. H. KIRTONG. L. BENNETT1

J. L. DOBBIEG. J. K. MERCERD. M. DUGANZICH

AgResearchRuakura Agricultural CentrePrivate Bag 3123Hamilton, New Zealand

1Present address: P. O. Box 166, Clay Centre,Nebraska 68933, United States.

Abstract An experiment was repeated in 2 yearsusing 698 lambs to investigate the effects of sirebreed (Southdown, Suffolk), sex (female, short-scrotum male), and growth path on lamb growthand carcass composition. The three growth pathsused were high (H), high followed by maintenance(HM), or low followed by high (LH) levels ofpasture availability. The Suffolk-cross lambsaveraged 2.3 kg more than Southdown-cross lambsat weaning (unfasted liveweight) and gained 2.6kg more than the Southdown-cross lambs betweenweaning and slaughter. The male lambs weighed1.8 kg more than the females at weaning and gained0.7 kg more between weaning and slaughter. Whenhot-carcass weight was adjusted to 17.1 kg, theSouthdown-cross lambs averaged 1.6 mm higherGR than Suffolks and females averaged 1.6 mmhigher GR than short-scrotum lambs. The effect ofthe different nutritional treatments applied in thisstudy was not consistent on GR or any other carcassmeasurements. The HM lambs had the largestcarcass measurements indicative of skeletal growthand the highest kidney fat weights. The results ofthis trial indicate that to produce lean heavy lambs,farmers should use rams from breeds with a large

A94009Received 16 February 1994; accepted 30 September 1994

mature size and feed only male lambs to heavierweights. Altering carcass composition throughnutritional manipulation is far less likely to producethe desired outcome.

Keywords sheep; Southdown; Suffolk; feeding;carcass composition; fat; GR; ewe lambs; short-scrotum male lambs

INTRODUCTION

In recent years, lamb producers have been paiddifferential premiums especially for heaviercarcasses exceeding 17 kg. Many of these heavycarcasses have an unacceptable level of fatnessplacing them in over-fat categories. Over-fatcarcasses are paid 25-50% less than carcasses ofacceptable fatness. To produce heavier carcassesthat would avoid over-fatness penalties, one NewZealand meat company recommended recon-ditioning lambs by feeding them to heavier weights,then severely under-feeding them for 7-10 days tocause a rapid liveweight loss believed to beassociated with leaner carcasses. The meat companybelieved that reaching any chosen carcass weightafter weight loss results in lower fat measurementsthan reaching these weights while the lambs arestill growing. Although there is little evidence toshow that underfeeding alone reduces carcassfatness on pasture-fed lambs in any appreciableway (Kirton et al. 1981, 1989; Thorrold et al.1988), Bray et al. (1988) showed that if combinedwith shearing, underfeeding can reduce GR levelsin cryptorchid lambs. GR is the fatness measure-ment specified for grading purposes (Kirton 1989),being the total tissue depth over the 12th rib, 11 cmfrom the mid-line. The Suffolk ram breed has beenshown to produce leaner lambs than Southdowns(Fahmy et al. 1972; Kirton et al. 1974; Wolf &Smith 1983; Kempster et al. 1986), and entiremales have been shown to be heavier and leanerthan ewe lambs (reviewed by Kirton & Morris1989).

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106 New Zealand Journal of Agricultural Research, 1995, Vol. 38

The present experiment aimed to look at theeffects of ram breed, sex, and ethically acceptabledifferent growth paths when combined in oneexperiment in one environment and repeated for 2years. This trial differs from previous trials ininvestigating the effects of three factors believedto influence lamb carcass fatness in combinationrather than individually to test if these effects wereadditive or less effective in combination.

MATERIALS AND METHODS

AnimalsData were available on 698 lambs born in 1985and 1986 and sired by Southdown and Suffolkrams. Ten rams per sire breed per year chosenfrom a wide selection of strain sources of bothbreeds were mated to three ewe genotypes(Romney, Border Leicester x Romney, and amixture of Coopworth and Coop worth x Romney).Male lambs were converted to short scrotum, thetesticles being placed against the body wall within24 h of birth and held in place by a rubber ringplaced over the scrotum (Probert & Davies 1986).

Slaughter treatmentsThe objective was to slaughter the lambs from thedifferent growth treatments at the same within-breed mean carcass weight. This was achieved byslaughtering the lambs on the highest planenutritional treatment at 25 weeks average age andthe lambs on the HM and LH plane of nutrition at31 weeks of age respectively but at the same targetliveweight.

Growth treatmentsTable 1 gives the numbers of lambs of each breedand nutritional treatment for which all measure-

ments reported were available for analysis. Year inthe Tables refers to birth year, lambs beingslaughtered the following year. From 3 weeks post-lambing, all ewes with lambs at foot had availablefor utilisation a visually estimated offer as reportedby Smeaton et al. (1981) of 1.5 kg DM/ewe perday which increased to reach a maximum feedoffer of 3.0 kg DM/day when the lambs were 8weeks old. This rate was maintained until the lambswere weaned at 12 weeks of age ± 4 days. Theincrease in assessed pasture allowance offer was aresult of pasture production caused by seasonaleffects.

The animals were moved to fresh grazing whenan estimated 40-50% of total dry matter (DM) hadbeen utilised. In early October of both years, thesward components were 55% grass, 35% clover,and 10% weeds. The sward remained green andleafy until weaning. Jagusch et al. (1979) andRattray et al. (1983) reported that herbageallowance, and not pasture yield, digestibility,accessibility to herbage, or season, was the majorfactor affecting growth rate. Herbage allowance isa commonly used assessment method in NewZealand farming for predicting the growth rateperformance of different classes of livestock andwas the basis to establish the different growthpaths for this trial. In addition to herbage allowance,the energy value (Mj ME/kg DM) of the pre-grazedsward was a contributing factor in determiningdaily feed allowance. The estimated values usedwere based on those for typical type pasturereviewed by Ulyatt et al. (1980) and were aconsensus of 2 and sometimes 3 operators. Thevalue of herbage allowance as a predictor for animalgrowth performance is greatest when the mass isutilised over 1 or 2 days (Jagush et al. 1979).Growth treatments were applied after weaning atan average age of 84 ± 4 days on to a similar

Table 1 Number of lambs used in this trial selected from those born in 1985and 1986.

Growthtreatment

Low-highHigh-maintenanceHighTotal

1985

Southdown

565757

170

Year of birth

Suffolk

606061

181

1986

Southdown Suffolk

585957

174

575858

173

Total

231234233698

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Kirton et al.—Factors affecting crossbred lamb composition 107

species mixed grass and clover pasture as wasgrazed pre-weaning.

The object was to slaughter the lambs on thedifferent growth treatments at the same within-sirebreed mean carcass weight and age. The growthpaths used were: high (H); high followed bymaintenance (HM); or low followed by high (LH),defined in terms of feed allowance. Feed allowancefor the H lambs from weaning to slaughter in midFebruary each year was 1.5 kg DM/head per day(c. 17 Mj ME/kg DM). The HM group was alsogrown in the same mob at the same rate to midFebruary. The LH lambs over the post-weaning tomid-February period were grazed in a separatemob on the same area as the H and HM groupsbeing separated by electrified netting. They wereoffered a feed allowance of 1.4 kg DM/head perday which contained an estimated 14 Mj ME/kgDM. From mid February until slaughter, the HMlambs immediately followed the grazing rotationof the LH lambs and mature cattle respectively.They were fed a maintenance diet consisting of afeed offer of 1.0 kg DM/head per day, attainingaverage (over the 2 years) full liveweights of 40.2and 34.8 kg respectively for Suffolk- andSouthdown-sired lambs at the beginning of April.As the green/dry fraction and the clover/grass ratioof the sward became increasingly variable frommid February to slaughter in early April because ofclimatic conditions, a subsample of animals fromeach treatment was weighed at 2-weekly intervals.Liveweight changes for the HM and LH groupswhich projected a different slaughter liveweight tothe H lambs were used to adjust the feed offer inconjunction with the estimated energy value. TheH lambs were on their post-weaning growth regimefor 99 days in 1985 and 94 days in 1986. The othertwo groups grazed for an additional 6 weeks eachyear.

Carcass measurement and analysesAfter dressing, the kidneys and kidney fat wereremoved from the hot-dressed carcass beforeweighing (HCW) and the GR measurement (Kirton1989) was taken in the chiller. After overnightchilling, the carcasses were measured for length(CARCL—Moxham & Brownlie 1976; gambrel tobeneath neck) and leg lengths F and T (Palsson1939) were taken. Carcasses were then sectionedbetween the last thoracic and first lumbar vertebraewith a knife following the curve of the last rib onboth sides of the carcass halfway to the flank. Thisexposed the cut surface of the longissimus thoracis

et lumborum (eye) muscle on both sides of thecarcass. Eye muscle width (A), depth (B), fat coverover B (C), and the thickest layer of fat over the rib(J) were measured as described by Palsson (1939)with J only being measured on the lambs born in1985.

For the lambs born in 1985, carcasses werehalved by cutting down the vertebral column, theleft side weighed, and then cut into DEVCo cuts(DEVCo 1984). Measurement S2 (Kirton et al.1967) was taken on the surface where the shoulderwas separated from the rib cut by a band-saw cutbetween the 5th and 6th ribs. GR2 was measured/estimated on the cut surface between the lastthoracic and first lumbar vertebrae with the cutcontinuing perpendicular to the vertebral column.Where this cut severed a rib at other than 11 cmfrom the mid-line, the rib thickness had to beestimated at the 11-cm site and hence an estimateof GR2 was obtained. Normally measured GR andGR2 are equivalent measurements (Kirton &Johnson 1977).

The left side was sectioned, minced (includingbone), and duplicate samples were taken todetermine water, ether-extract, protein (dried, fat-free, ash-free residue), and ash contents (Kirton etal. 1962).

Statistical analysisPreliminary testing for interactions was carried outby least squares analysis of variance. Only theinteraction of year by sire breed by growth pathwas consistently significant. For variates availablein both years, the means presented are the bestlinear unbiased estimates resulting from usingrestricted maximum likelihood (REML), with amodel comprising year, sire breed, and growthpath with all their interactions, sex and birth/rearingrank of lamb, age and genotype of dam, residualsire effect (i.e. sire within year by sire breedsubclass) as a random effect, plus relevantcovariates which are noted in the Results section.The REML model obtained by omitting year wasused for the other variates.

The within-cross weaning weight covariate usedfor the estimates given in Table 2 was calculatedby difference from the appropriate sire breed bydam breed combination means obtained from theanalysis of weaning weight. It was used to avoidaffecting variation associated with breed.

Least square analyses were carried out with theregression procedure in the Genstat statisticalpackage (Lawes Agricultural Trust 1984), and

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108 New Zealand Journal of Agricultural Research, 1995, Vol. 38

40"

38-

36"

34"

.g> 32"

28"

26

24Wean Nov Dec Jan Feb Mar Apr

Fig. 1 Adjusted liveweight change from weaning toslaughter averaged over 1985 and 1986 birth years forhigh (—•—) High-maintenance (—O—) and low(—A—) growth path lambs.

REML with the REML program (Patterson &Thompson 1971).

RESULTS

Mean adjusted growth rates from birth to weaningfor the Suffolk- and Southdown-sired lambsrespectively, were 264 and 241 g/day averagedover the 2 years. The Suffolk-cross lambs averaged

2.3 kg heavier in full liveweight at weaning thanthe Southdown-cross lambs. Adjusted growth ratesfor the 1985-born H and HM groups from weaningto mid February were 140 and 120 g/day andSuffolk- and Southdown-cross lambs attained fullliveweights of 41.7 and 36.7 kg respectively.Average adjusted growth rates from weaning toslaughter for the 1985-born LH Suffolk andSouthdown lambs were 99 and 81 g/day attaining aslaughter liveweight of 41.1 and 35.6 kgrespectively. The same growth patterns werefollowed in both years, except that slaughter weightwas lower for the 1986-born lambs. On average,the Suffolk-cross lambs gained 2.6 kg more thanthe Southdown-cross lambs between weaning andslaughter. Fig. 1 shows that for both years, lambsin all treatments gained little liveweight for 4 weekspost-weaning. This trend continued among the LHlambs until January. A statistical model includingsex, sire breed, dam breed, birth weaning rank, ageof dam, and birthday and year was used to estimateeach of the points in Fig. 1. HM Suffolk- andSouthdown-sired lambs born in 1985 attained fullliveweights of 43.2 and 37.0 kg and were 14 and12% heavier respectively than those born in 1986(Table 2). Starved liveweights and carcass weightsof 1985-born lambs were 2.7 and 1.6 kg heavierthan those of the 1986-born lambs respectively(Table 2). The Suffolk-cross lambs producedheavier carcasses when slaughtered at the sametime, with these carcasses containing less fat whencompared on a weight basis.

Table 2 Full liveweights (FLW, kg), starved liveweights (SLW, kg) and hot-carcass weights (HCW, kg) ofSouthdown- and Suffolk-cross lambs fed to grow on different growth paths (adjusted for variation in weaning ageand within-cross weaning weight).

Growthtreatment

FLW (kg)

Year Southdown Suffolk

SLW (kg)

Southdown Suffolk

HCW (kg)

Southdown Suffolk

Low-highHigh-maintenanceHighLow-highHigh-maintenanceHigh

Growth SED1

Other SED2

1985

1986

35.637.036.7

35.332.533.5

0.530.57

41.143.236.739.937.137.7

32.634.133.2

31.630.231.6

37.439.533.235.434.235.2

0.490.50

16.017.217.0

15.914.915.8

0.280.30

18.419.619.2

17.516.417.2

SED = standard error of difference'Within breed and year2A11 other comparisons

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Kirton et al.—Factors affecting crossbred lamb composition 109

Growth treatments within sire breed did notdiffer in starved liveweights. However, fullliveweights for both sire breeds were more variable,particularly among the 1986-born lambs (Table 2).The H group pooled across years, had carcassesthat were 0.35 kg heavier than the LH animals and0.27 kg heavier than the HM lambs. There was agrowth treatment by birth year interaction (P < 0.01)for all variables in Table 2.

The effects of breed and growth treatments onGR, C, and kidney fat weight over 2 years arereported in Table 3 for lambs compared afteranalysis by covariance correcting to the samecarcass weight (17.1 kg). This analysis wasundertaken to separate the breed effect on carcassweight from the breed effect on carcass com-position. The 1985-bom lambs averaged 1.5 mmlarger GR values than that measured in the 1986-born lambs. The faster growing H 1985-born lambshad higher GR values whereas in 1986-born lambsthe same treatment had lower GR depth measure-ments resulting in a significant interaction(P < 0.01) and an overall lack of growth treatmenteffect on this measurement. The Suffolk-crosslambs averaged 1.6 mm lower GR than theSouthdown-cross lambs when compared at the samecarcass weight.

Lambs born in 1985 had a 1.6 mm higheraverage C measurement than the 1986-born lambsafter adjustment to the same carcass weight. Forthe 1986-born lambs, there was atrend of increasingC measurement from the LH to the H nutritiontreatments, but this did not apply for the 1985-bornlambs, so that overall there was a growth pathtreatment effect (P < 0.01) of 0.46 mm C between

the high and low treatments; there was also a growthtreatment by year interaction (P < 0.01). TheSuffolk-cross lambs averaged 0.42 mm lower Cthan the Southdown-cross lambs.

Birth year did not influence kidney fat weight(Table 3). Although the growth treatment affectedkidney fat weight (P < 0.01), highest mean weightswere found for the HM growth treatment (exceptfor the Suffolk in 1986) with little differencebetween the H and LH groups in contrast with theapparent growth treatment effects on the sub-cutaneous fat depths (Tables 3 and 4). As with allother fatness measurements, the Southdown-crossmean kidney fat weights were higher than thosefor the Suffolk-cross lambs when compared at thesame mean hot-carcass weight.

Table 4 shows some treatment effects on somefat measurements recorded only for the 1985-bornlambs. The Southdown-cross lambs were fatterthan the Suffolk-cross lambs for all subcutaneousfat measurements reported as well as for percentagecarcass fat. The growth treatments did not differ inJ or percentage carcass fat. The S2 means increasedfrom the low through to the other two nutritionaltreatments.

The effect of sire breed and growth treatmenton carcass length and leg length measurements Fand T is reported in Table 5. The means for each ofthe three measurments were similar in both years,and in line with previous unpublished and other(Fahmy et al. 1972) observations. The lengthmeasurements taken on the Suffolk-cross carcasseswere larger than those on the Southdown-crosscarcasses. The growth treatment effect was highlysignificant (P < 0.01) for all length measurements,

Table 3 Measurements GR (mm), C (mm), and kidney fat weight (KFW, g) of Southdown- and Suffolk-crosslambs fed to grow on different growth paths adjusted to 17.1 kg carcass weight (by covariance).

GR (mm) C (mm) KFW (g;Growthtreatment

Low-highHigh-maintenanceHigh

Low-highHigh-maintenanceHigh

Growth SED1

Other SED2

Year

1985

1986

Southdown

11.812.813.6

11.411.110.2

0.440.53

Suffolk

10.710.411.3

10.29.59.1

Southdown

4.184.994.49

2.713.023.33

0.250.31

Suffolk

4.214.344.14

2.112.313.11

Southdown

332357332

326344327

1619

Suffolk

318335275

309304294

'Within breed and year2 A11 other comparisons

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110 New Zealand Journal of Agricultural Research, 1995, Vol. 38

with a significant growth treatment by yearinteraction (P < 0.05) only for CARCL. Lengthmeasurements were greatest for the HM treatments.

Table 6 reports the mean width (A) and depth(B) measurements of the eye muscle recorded atthe loin/thoracic interface as well as the proportionof protein in the carcasses only in the 1985 data.Eye muscles were wider for the 1986-born lambs.Neither breed nor growth treatment had any effecton eye muscle width. The Southdown-cross lambshad deeper eye muscles than the Suffolk-crosslambs in line with the normally observed greatereye muscle areas found in Southdown-cross lambsat the same carcass weight. Lambs on the Htreatment had the smallest eye muscle depth,possibly resulting from being 6 weeks younger atslaughter than other lambs. The HM and LH lambsswitched rankings over the 2 years resulting in agrowth treatment by year interaction (P < 0.01).

Over breeds, the short-scrotum male lambs were2.2 kg heavier than ewe lambs in full liveweight at

weaning and gained 0.6 kg more between weaningand slaughter. Table 7 reports the effect of lambsex on several measures of fatness after adjustingfor other experimental factors by covariance to17.1 kg carcass weight. These means confirm thatthe carcasses of the short-scrotum male lambs were,on average, lower for all fatness measurementsthan the female lambs, in particular averaging 1.9mm lower in GR and 2% lower in the carcass fat atthe same carcass weight. The 1986 carcasses wereleaner than those in 1985 for GR and C but notkidney fat.

If no statistical corrections are considered forany of the factors measured in this trial, the Suffolkshort-scrotum lambs averaged 3.2 and 1.5 kgheavier in hot-carcass weight than the Southdownewe and short-scrotum lambs respectively at thesame overall age at slaughter (Table 8). Despitethis heavier carcass weight, these Suffolk malelambs still had similar mean GR levels to bothSouthdown sexes at the actual mean carcass weights

Table 4 Measurements GR2 (mm), J (mm), S2 (mm), and percentage carcass fat of Southdown and Suffolk crosslambs fed to grow on different growth paths in one season (1985-born) and adjusted to 17.1 kg carcass weight bycovariance.

GR2 (mm) J (mm) S2 (mm) %fatGrowthtreatment

Low-highHigh-maintenanceHigh

Growth SED'Breed2

'Within breed2Between breeds

Southdown

14.116.115.2

0.590.69

Suffolk

13.413.713.4

Southdown

13.413.014.2

0.0.

Suffolk

12.111.711.6

5270

Southdown Suffolk

8.29.39.5

7.17.97.5

0.380.47

Southdown

30.731.731.8

0.490.68

Suffolk

28.929.128.7

Table 5 Carcass (CARCL, cm) and leg length measurements (F, mm; T, mm) of Southdown- and Suffolk-crosslambs fed to grow on different growth paths adjusted to 17.1 kg carcass weight by covariance.

Growthtreatment

Low-highHigh-maintenanceHigh

Low-highHigh-maintenanceHigh

Growth SED1

Other SED2

Year

1985

1986

CARCL (cm)

Southdown

93.094.393.4

94.294.993.1

0.360.57

Suffolk

96.097.996.6

96.998.396.5

F (mm)

Southdown Suffolk

225.8 248.2229.9 254.7223.3 248.4

234.6 255.9237.8 263.0230.0 253.8

1.892.79

T' (mm)

Southdown Suffolk

171.6173.5170.9

177.6179.6176.9

184.5187.8184.3

189.5193.1190.4

1.051.65

'Within breed and year2A11 other comparisons

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Kirton et al.—Factors affecting crossbred lamb composition

for the slaughter groups indicating advantages from DISCUSSIONthe use of Suffolk rams if producers wish to produceheavyweight lean lambs. These data without thestatistical corrections for weaning weight and hot-carcass weight indicate the level of differencesproducers might achieve when slaughtering lambsof different breeds and sexes at the same time(age).

I l l

The results presented in this paper investigate theadditional contribution that nutritional plane canmake to known effects of sire breed and sex tolamb carcass weight and fatness. Greater growthrates for lambs born in 1985 than those born in1986 is likely to be the result of climatic differences

Table 6 Muscle measurements A (mm) and B (mm) and carcass percentage protein content (one season) of lambsfed to grow on different growth paths adjusted to 17.1 kg carcass weight by covariance.

Growthtreatment

Low-highHigh-maintenanceHigh

Low-highHigh-maintenanceHigh

Growth SED1

Other SED2

Year

1985

1986

i\ (mm)

Southdown \

52.351.752.5

54.554.653.9

0.550.73

Suffolk

50.952.552.2

54.755.053.2

B (mm)

Southdown

30.632.129.2

30.228.929.1

0.380.47

Suffolk

28.829.827.7

30.529.228.0

% protein

Southdown Suffolk

15.4 15.915.0 15.814.9 15.8

-

0.140.17

'Within breed and year2AU other comparisons

Table 7 Effect of sex (short scrotum, female) on fatness measurement GR (mm), C (mm), andkidney fat (g) weight in two seasons and percentage carcass fat (1985) adjusted to 17.1 kg carcassweight by covariance.

Growthtreatment

MaleFemale

Sex SEDYear SED

GR (mm)

1985 1986

10.8 9.512.7 11.0

0.260.36

C(mm)

1985 1986

4.02 2.604.68 2.98

0.150.21

Kidney f?at (g)

1985 1986

280 286366 351

9.513.4

c f a t

1985 1986

29.231.1

0.30

Table 8 Effect of breed, sex, and growth path on actual (uncorrected) means of hot-carcassweight (kg) and GR (mm) at slaughter averaged over years.

Growthtreatment

Low-highHigh-maintenanceHigh

Overall

Southdown

HCW

17.216.717.2

17.0

GR

11.110.811.6

11.2

Male

Suffolk

HCW

18.218.418.9

18.5

GR

11.010.511.1

10.9

Female

Southdown

HCW

14.915.115.7

15.3

GR

10.110.911.3

10.8

Suffolk

HCW

17.317.718.0

17.7

GR

11.711.812.5

12.0

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112 New Zealand Journal of Agricultural Research, 1995, Vol. 38

affecting pasture palatability and/or a ram effect asa result of a small number of rams per breed matedper year. The poor initial post-weaning growthrates of the H and HM lambs could result from theimmediate pre-grazing pasture preparation by adultcattle lowering feed palatability.

Reports from previous trials (Fahmy et al. 1972;Kirton et al. 1974; Wolf & Smith 1983; Kempsteret al. 1986) have shown that at the same carcassweight, lambs sired by the Southdown are fatter onaverage than those sired by the Suffolk. Whencompared at the same level of carcass fatness,Southdown-cross carcasses were lighter thanSuffolk-cross carcasses (Read 1982). Both methodsof evaluation indicated that the Southdown is abreed of smaller mature size (see Kirton & Morris1989). In the present experiment, the crossbredlambs were compared for composition at the samecarcass weight and the results confirmed that sirebreed is an important option for changing lambcarcass size and fatness. These results reflect thelarger mature size of the Suffolk-cross lambs and ithas been shown that breed differences incomposition at the same weight largely disappearwhen the comparisons are made on the basis of thesame proportion of mature size (McLelland et al.1976; Kirton & Morris 1989). The deeper eyemuscles (Measurement B) of the Southdown-crosslambs compared to the Suffolk-cross lambs at thesame weight is an expected observation (Kirton etal. unpubl. data).

The Southdown-cross carcasses were shorterthan the Suffolk-cross carcasses of similar weight.The shorter carcasses provide a blocky conform-ation that is desirable for some markets and theassociated deeper eye muscles provide an attributethat is also a desirable characteristic whenmarketing lamb meat, provided this can be achievedwithout excessive fatness (Kempster et al. 1982).Other published results have shown that these sirebreed effects can be further enhanced by pickingthe genetically leaner rams within any sire breed(Kadimetal. 1989).

The other well-established method of producingheavier and leaner lambs is to use the greater maturesize of male sheep. Ewe and wether lambs growmore slowly and will be fatter at any carcass weightthan the entire male or cryptorchid or short-scrotumram (reviewed by Kirton & Morris 1989). Thefatness differences observed for the two sexes inthe present experiment were consistent over thetwo seasons and were at the lower end of thedifferences previously reported.

Lambs on the high growth path were 6 weeksyounger at slaughter in comparison with the othertwo growth treatments. If all lambs had beenslaughtered at the same time, the H animals wouldhave had the opportunity to accumulate a further 6weeks growth. The further 6 weeks of growthrestriction was applied to the HM lambs becauseof the possibility that this might restrict fatdeposition below the levels associated with highgrowth. The results from this trial, in line with theresults of earlier trials (Kirton et al. 1981, 1989;Bray et al. 1988; Thorrold et al. 1988) indicate thatattempting to alter carcass composition throughthe manipulation of pasture intake is unpredictable.Whereas the H lambs had the highest GR values inthe first season, they had the lowest values in thesecond season. The differences were small (2.7and 7%) for Southdown- and Suffolk-sired lambsrespectively and probably would not haveinfluenced carcass grade. Although the two trialsdiffered with their pre-slaughter liveweighttrajectories, the responses to treatment agree withThatcher (1992) who reported pasture-grazed HLlambs had greater GR and kidney fat measurementsplus a smaller eye muscle area (cf. MeasurementsA, B, Table 6), than LH lambs. Butler-Hogg (1984)and Butler-Hogg & Johnsson (1986) however,found pen-fed lambs of their LH group containedcarcasses with more fat and less water. Duration ofthe re-alimentation stage and differences in trialdesign before slaughter could be contributingfactors to the different results. Relative to this trial,those of Butler-Hogg (1984) and Butler-Hogg &Johnsson (1986) had a duration weight loss of 18and 16 weeks respectively followed by a rapid re-alimentation period of c. 7 weeks. In the weightloss/gain phase 2 period of the trial reported byThatcher & Gaunt (1992), the duration was 4 weeks.A smaller eye muscle area, or B measurement wasconsistent for high plane animals for all three trials.Evidence from other carcass development studiesat the Ruakura Agricultural Centre (Clarke unpubl.data) of which this trial is a subset, supports thenegative effect liveweight loss has on eye musclearea. Pre- and post-grazing measurement of pastureintake and of forage quality may also assist similarexperiments with the interpretation of results. Thebetween-year effect of sire on carcass traits is notknown. It is important to decide whether or not inclu-sion of link sires is part of the experimental design.

One unexpected result in the present trial wasthe greater leg and carcass length measurements ofthe HM lambs when contrasted with the other two

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growth treatments. The H lambs may be assumedto have the same length measurements as the HMlambs at the time the H lambs were slaughteredand therefore the additional 6 weeks of maintenancewould provide a reasonable explanation for thegreater length measurements of the HM lambs.These results support those of Butler-Hogg &Johnsson (1986) who reported greater leg andespecially carcass length measurement (P < 0.001)among HL lambs when restricted in feed intakebetween 20 and 36 weeks of age. Hopkins & Tulloh(1985) reported that lambs severely restricted forthe first 5 weeks of post-natal life (R) and thengrown on the same high-quality feed for 27 dayslonger than a control group (C) had slightly longerleg bones (small numbers and not significant). Thelower feed levels of the R lambs at the beginningof that trial with its potential to restrict bonedevelopment had apparently contributed to a meanage difference between the groups of 27 days atslaughter. The greater weights of kidney fat of theHM lambs relative to the other groups wereconsistent over both years and for the 1985-bornlambs is supported by carcass chemical percentagefat (Table 4).

In summary, the results of this trial and ofother publications cited indicate that lamb producerswishing to produce heavier, leaner lambs will havemore success by managing their male lambs toreach heavier weights and slaughtering their ewelambs at lighter weights. They will also have moresuccess if they choose rams from the larger maturesize sheep breeds as flock sires. The breed effectwill be enhanced if they select from the geneticallyleaner sires within breeds. The results confirm thatof this and other pasture and/or pen-based trials,the GR response to nutritional manipulation ofgrowth rates is unpredictable.

ACKNOWLEDGMENTS

We thank staff at the Waikeria Youth Centre whomanaged the lambs in this trial; staff at the RuakuraAbattoir who slaughtered and measured these lambs;and chemical servicing staff who undertook carcasschemical analysis.

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Butler-Hogg, B. W. 1984: Growth patterns in sheep:changes in the chemical composition of theempty body and its constituent parts duringweight loss and compensatory growth. Journalof agricultural science, Cambridge 105: 17-24.

Butler-Hogg, B. W.; Johnsson, I. D. 1986: Fatpartitioning and tissue distribution in crossbredewes following different growth paths. Animalproduction 42: 65-72.

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